This is preparatory commit for #18941.
Importing `do_delete_message` from `message_edit.py` was causing a
circular import error. In order to avoid that, we create a separate
message_delete.py file which has all the functions related to deleting
messages.
The tests for deleting messages are present in
`zerver/tests/test_message_edit.py`.
Fixes a part of #18941
madrix01